Method of controlling operational venting of a plastics fuel tank and plastics fuel tank

ABSTRACT

The invention relates to a method of controlling operational venting of a plastics fuel tank, in which the volume of the plastics fuel tank is kept within a predetermined differential pressure range relative to ambient pressure by means of at least one operational venting valve with pressure maintaining function, at least an upper limit of the differential pressure range defining a maximum admissible overpressure being varied as a function of the ambient temperature.

The invention relates to a method of controlling operational venting of a plastics fuel tank and to a plastics fuel tank for a motor vehicle with means for inward and outward venting during fuel discharge (operational venting), having at least one operational venting valve between at least one operational venting point and a fuel vapor filter, the operational venting valve taking the form of a pressure maintaining valve.

In modern passenger cars with hybrid drive designs, comprising a combustion engine and one or more electric motors, fuel tanks are increasingly being used which have non-integrated venting systems. Non-integrated venting systems normally use separate venting paths for operational venting and refueling venting of the fuel tank. Operational venting of a fuel tank means removing from the tank fuel vapor which arises as a result of surging movements of the fuel, caused by the vehicle's motion and temperature changes, whereas refueling venting of the fuel tank involves conveying the gas volume displaced by the inflowing liquid out of the tank into the atmosphere via a fuel vapor filter, the fuel vapor filter, which conventionally takes the form of an activated carbon filter, becoming fouled with hydrocarbons and waste gas from which all hydrocarbons have been removed being released into the atmosphere. Defouling or back-flushing of the fuel vapor filter normally takes place during driving cycles of the motor vehicle, the combustion air for the internal combustion engine being drawn in via the fuel vapor filter, so resulting in defouling and back-flushing of the fuel vapor filter.

In hybrid vehicles these possible back-flushing cycles of the motor vehicle are reduced by the number of times in which the motor vehicle is driven electromotively. Consequently, the fuel vapor filters provided for this purpose have to have a greater capacity, or precautions have to be taken to reduce the transition of liquid hydrocarbons into the gaseous phase.

For this reason, fuel tanks for hybrid applications are often produced in the form of pressure tanks with non-integrated venting systems. To this end, the venting valves take the form of pressure maintaining valves, which are electrically switchable in order to be able to bring about pressure equalization with the atmosphere prior to initiation of a refueling process. This is conventionally achieved by electrically switched venting valves. The overpressure of the fuel tank admissible as a function of the design of the fuel tank may be for example of an order of 400 mbar, the maximum admissible partial vacuum being around 100 mbar. When using steel tanks, this is not a problem provided steel tanks of sufficient material thickness are used. This goes hand in hand, however, with the disadvantage of a marked increase in weight. However, when using plastics fuel tanks, deformation of the tank bladder may arise under high pressure depending on the geometry of the fuel tank. This is often counteracted by banding, reinforcing profiles and the like. Such measures are structurally very complex, and moreover do not take account of the fact that the deformation behavior of plastics or the plasticity thereof is temperature-dependent. A plastics fuel tank may lose part of its inherent stability when given temperatures are exceeded. The known reinforcing measures take account of this only to a certain degree.

It is accordingly an object of the invention to provide a method of controlling operational venting of a plastics fuel tank which largely avoids the above-described disadvantages. It is also an object of the invention to provide a corresponding fuel tank.

The object is initially achieved by a method of controlling operational venting of a plastics fuel tank in which the volume of the plastics fuel tank is kept within a predetermined differential range relative to ambient pressure by means of at least one operational venting valve with pressure maintaining function, at least an upper limit of the differential pressure range defining a maximum admissible overpressure being varied as a function of the ambient temperature.

In other words, the invention proposes ambient temperature-dependent pressure regulation of the fuel tank, whereby account is reliably taken of the changes to the inherent rigidity of the fuel tank arising as a function of the ambient temperature.

The term “ambient temperature” may be understood for the purposes of the present invention as also meaning the system temperature, i.e. the temperature of the fuel tank determined for example by the conveyance of hot diesel fuel. The fuel in a number of fuel feed systems with fuel return is likewise known to undergo a degree of heating, which contributes to heating of the overall fuel tank system.

In a preferred variant of the method according to the invention, the upper limit of the differential pressure range is lowered if the ambient temperature exceeds at least a predetermined upper limit, and the upper limit of the differential pressure range is raised if the ambient temperature falls below at least a predetermined lower limit. The upper and lower limits may coincide. However, a lower limit may be provided which deviates from the upper limit, so as to provide switching hysteresis for control system purposes.

A preferred variant of the method provides the use of at least one operational venting valve with a temperature-dependent opening and closing characteristic.

The admissible differential pressure relative to ambient temperature may amount to between 0 and 500 mbar.

The method is particularly preferably performed using an operational venting valve switchable without current and in a temperature-dependent manner.

The object underlying the invention is additionally achieved by a plastics fuel tank with means for inward and outward venting during fuel discharge (operational venting), having at least one operational venting valve between at least one operational venting point and a fuel vapor filter, the operational venting valve taking the form of a pressure maintaining valve and the plastics fuel tank being distinguished in that the operational venting valve has an ambient temperature-dependent switching characteristic.

The invention should be understood such that the design of the plastics fuel tank may exhibit any desired degree of complexity, wherein operational venting points may be provided at different levels depending on the contour of the plastics fuel tank at a plurality of locations at the top in the installation position. Operational venting systems of the above-described type may be provided at each venting point or also in a central venting manifold.

In a convenient configuration of the plastics fuel tank according to the invention, the operational venting valve takes the form of a valve which switches automatically without current. This configuration is particularly advantageous when venting control is to be performed without the vehicle battery. It is conceivable, for example, for the vehicle to be exposed to relatively high ambient temperatures in a repainting chamber, for example, without its battery installed.

In one advantageous variant of the plastics fuel tank, the valve spring may consist for example of a shape memory alloy, i.e. a “memory metal”. These are alloys which are seemingly able to “remember” an earlier shape despite subsequent severe deformation. A change in shape, once imposed on the material, may be undone for example by heating. The materials used here are then conveniently those which allow a 2-way memory effect, i.e. said metals return to the initial shape on cooling.

In a particularly advantageous configuration of the plastics fuel tank according to the invention, the operational venting valve comprises at least one valve body held in the closed position under spring loading, at least one valve spring having a temperature-dependent spring constant.

The closure force of the operational venting valve may amount to around 100% for example up to an ambient temperature or system temperature of around 50° C., which may correspond to a maximum admissible overpressure of around 400 mbar relative to atmospheric pressure. At an ambient temperature or system temperature of around 70° C. or of over 70° C., the closure force of the venting valve may approach 0%, for example, such that the fuel tank is in effect pressureless.

The valve spring may take the form of a bimetallic spring. The spring constant of the valve spring varies in accordance with ambient temperature, whereby a temperature-dependent switching characteristic is achieved with the simplest means.

In a convenient configuration of the plastics fuel tank according to the invention, the latter comprises a refueling venting means which makes use of a switchable venting path separate from the operational venting.

The plastics fuel tank according to the invention conveniently takes the form of a plastics fuel tank for a hybrid motor vehicle.

An advantageous variant of the plastics fuel tank according to the invention and the method according to the invention are explained hereinafter with reference to an exemplary embodiment illustrated in the drawings, in which:

FIG. 1 is a schematic, highly simplified representation of a fuel tank according to the invention and

FIG. 2 is a highly simplified, schematic representation of an operational venting valve according to the invention.

The fuel tank 1 according to the invention takes the form of a plastics fuel tank and is provided in a known manner with a filler neck 2, a refueling vent line, not shown, and at least one operational vent line 3. The operational vent line 3 communicates via one or more venting points 4 with the equalizing volume 5 of the fuel tank, which is naturally located at the top in the installation position. The equalizing volume 5 is the free gas volume in the fuel tank 1 situated above the liquid volume 6, which ensures that no liquid hydrocarbons enter the venting system in the event of a change in the volume of the fuel.

The operational vent line 3 connected to the equalizing volume 5 is connected to a fuel vapor filter 7 via an operational venting valve 6. The fuel vapor filter 7 may be constructed in a known manner as an activated carbon filter with one or more volumes containing sorbents.

As will be explained in greater detail below, the operational venting valve 6 takes the form of a valve with pressure maintaining function, such that it is ensured that a certain differential pressure range relative to ambient pressure is maintained within the plastics fuel tank 1. This serves in particular to reduce the tendency of the liquid hydrocarbons to pass into the gaseous phase. In this way it is ultimately possible, in particular in hybrid vehicles, to provide fuel vapor filters with a fouling capacity which does not fundamentally exceed the fouling capacity of a conventional fuel vapor filter despite shorter defouling cycles.

It is obvious to a person skilled in the art that the fuel tank 1 according to the invention may be provided with an operational venting valve at each of a plurality of venting points 4.

FIG. 2 is a simplified representation of an example of such an operational venting valve 6.

This comprises an outward venting path 8 and an inward venting path 9. The outward venting path 8 is closed by means of an outward venting valve 10 and the inward venting path 9 by means of an inward venting valve 11. The outward venting valve 10 comprises a valve body 10 a, which is held in the closed position by means of a bimetallic spring 10 b. The bimetallic spring 10 b has a spring constant which is designed such that the valve body 10 a is lifted by an overpressure of ≧400 mbar in relation to ambient pressure acting from the side of the equalizing volume 5 off the valve seat designated 10 c and opens up the operational vent line 3 as far as the fuel vapor filter 7. If the ambient temperature rises above a predetermined value, the spring constant of the bimetallic spring 10 b changes such that the valve body 10 a is lifted from the valve seat 10 c at a lower internal tank pressure, in order to counteract temperature-induced deformation of the plastics fuel tank 1.

The inward venting valve 11 provided in the parallel inward venting path 9 likewise comprises a valve body 11 a, which takes the form of a ball. The valve body 11 a is pressed by means of a compression spring 11 b into the valve seat 11 c, this corresponding to the normal position of the inward venting valve. The inward venting valve 11 opens for example under an atmospheric overpressure of around 100 mbar or with a partial vacuum in the tank of around 100 mbar.

It is in principle possible for the outward venting valve also to display an ambient temperature-dependent switching characteristic.

It is then provided according to the invention that a differential pressure range of around −100 mbar to +400 mbar, relative to atmospheric pressure, is maintained inside the plastics fuel tank 1 by means of one or more operational venting valves 6, configured as described above. In this case, the closing force of the outward venting valve 10 determines the maximum admissible upper limit of the differential pressure range. On the other hand, the closing force of the inward venting valve 11 determines the maximum admissible lower limit of the differential pressure range. As already explained above, the upper limit is variable as a function of ambient temperature due to the properties of the bimetallic spring 10 b of the outward venting valve 10. At a relatively high ambient temperature the pretension, with which the valve body 10 a is pressed into the valve seat 10 c, is lower, such that in this way temperature-dependent control functioning without current is brought about.

It is furthermore possible to provide the electrical pressure switching functions necessary for the system by way of electrical heating of the described spring element (e.g. bimetal or “memory metal”). Heating may here proceed by energizing the spring element itself and/or by an additional heating element.

LIST OF REFERENCE SIGNS

-   1 Plastics fuel tank -   2 Filler neck -   3 Operational vent line -   4 Venting point -   5 Equalizing volume -   6 Operational venting valve -   7 Fuel vapor filter -   8 Outward venting path -   9 Inward venting path -   10 Outward venting valve -   10 a Valve body -   10 b Bimetallic spring -   10 c Valve seat -   11 Inward venting valve -   11 a Valve body -   11 b Compression spring -   11 c Valve seat 

1. Method of controlling operational venting of a plastics fuel tank, in which the volume of the plastics fuel tank is kept within a predetermined differential pressure range relative to ambient pressure by means of at least one operational venting valve with pressure maintaining function, at least an upper limit of the differential pressure range defining a maximum admissible overpressure being varied as a function of the ambient temperature.
 2. Method according to claim 1, characterized in that the upper limit of the differential pressure range is lowered if the ambient temperature exceeds at least a predetermined upper limit, and the upper limit of the differential pressure range is raised if the ambient temperature falls below at least a predetermined lower limit.
 3. Method according to claim 1, characterized by the use of at least one operational venting valve with a temperature-dependent opening and closing characteristic.
 4. Method according to claim 1, characterized in that the admissible differential pressure (AT) may amount to between 0 and 500 mbar.
 5. Method according to claim 2, characterized in that a temperature of 70° C. is selected as the predetermined upper limit for the ambient temperature.
 6. Plastics fuel tank for motor vehicles with means for inward and outward venting during fuel discharge (operational venting), having at least one operational venting valve between at least one operational venting point and a fuel vapor filter, the operational venting valve taking the form of a pressure maintaining valve, characterized in that the operational venting valve has an ambient temperature-dependent switching characteristic.
 7. Plastics fuel tank according to claim 6, characterized in that the operational venting valve takes the form of an automatically regulating valve operating without current.
 8. Plastics fuel tank according to claim 6, characterized in that the operational venting valve comprises at least one valve body held in a closed position under spring loading, at least one valve spring having a temperature-dependent spring constant.
 9. Plastics fuel tank according to claim 6, characterized in that the valve spring takes the form of a bimetallic spring.
 10. Plastics fuel tank according to claim 6, characterized in that the valve spring consists of a shape memory alloy.
 11. Plastics fuel tank for a hybrid motor vehicle according to claim
 1. 